Physical activity is the leading health indicator in Healthy People 2010, a national agenda for reducing the most significant preventable threats to health. The scientific evidence is strong that regular physical activity—even at moderate levels, such as walking briskly for 30 minutes on 5 or more days per week—reduces the risk of premature mortality and the development of numerous chronic diseases, improves psychological well-being, and helps prevent weight gain and obesity by keeping caloric intake in balance with energy expenditure. Yet despite the scientific evidence, Americans have not taken sufficient initiative to meet federal guidelines on appropriate levels of total daily physical activity. Fully 55 percent of the U.S. adult population fall short of the guidelines, and approximately 25 percent report being completely inactive when not at work. Nearly one-third of high-school-age teenagers report not meeting recommended levels of physical activity, and 10 percent classify themselves as inactive. No corresponding summary assessment exists for children.
STUDY CONTEXT AND CHARGE
Over the past half-century or longer, major technological innovations—automation and the consequent decline of physically active occupations, labor-saving devices in the home, and the dominance of the automobile for personal travel—have substantially reduced the physical requirements of daily life. In addition, the steady decentralization of metropolitan area population and employment
to low-density, widely dispersed suburban locations has increased travel distances to many destinations (e.g., schools, neighborhood shopping, transit stops) and made the private vehicle the most practical and convenient transport mode. Lifestyle and cultural changes, such as increases in television watching and other sedentary activities, have also played a role in reducing physical activity.
The built environment has recently come under scrutiny as an important potential contributor to reduced levels of physical activity. The purpose of this study is to contribute to the debate on this issue by examining the role of land use and travel patterns in the physical activity levels of the U.S. population. The charge to the study committee was to review the broad trends affecting the relationships among physical activity, health, transportation, and land use; summarize what is known about these relationships, including the strength and magnitude of any causal connections; draw implications for policy; and recommend priorities for future research. The built environment is broadly defined to include land use patterns, the transportation system, and design features that together provide opportunities for travel and physical activity.1 Physical activity is defined as bodily movement produced by the contraction of skeletal muscle that increases energy expenditure above the basal level.
The built environment can be studied at various geographic scales—from the building and site to the neighborhood and regional levels. The focus of this study is primarily at the latter two levels; very little is known about physical activity at the building or site level. For the purposes of this study, physical activity is categorized into four types: leisure time or recreational, transportation, household, and occupational. The committee’s interest is in the effect of the built environment on overall physical activity because total daily physical activity levels are what matter from a public
health perspective, not whether an individual drives rather than walks or cycles on particular trips.
BENEFITS OF PHYSICAL ACTIVITY
The primary motivation for recent concern about inadequate levels of physical activity derives from the well-established, scientifically based causal connection between physical activity and health, as articulated in the U.S. Surgeon General’s first report on Physical Activity and Health in 1996. That report and the results of subsequent research confirm that regular physical activity reduces the risk of premature mortality from all causes. Moreover, regular physical activity reduces the risk of developing several leading chronic illnesses, including cardiovascular disease (e.g., heart attacks, strokes), colon cancer, and non-insulin-dependent diabetes, as well as their precursors (e.g., high blood pressure, hypertension). Other benefits of physical activity include reductions in the risk of developing obesity, osteoporosis, and depression, and improvements in psychological well-being and quality of life.
Concern about low levels of physical activity stems from economic considerations as well. According to the Centers for Disease Control and Prevention, the direct medical expenses associated with physical inactivity totaled more than $76 billion in 2000. This figure does not take into account indirect costs, such as lost productivity from the physical and mental disabilities to which sedentary behavior contributes.
The problem of inadequate physical activity is frequently and mistakenly confused with obesity, particularly in the popular press. The recent marked rise in obesity levels among the U.S. population—a major public health concern—is due to an energy imbalance. Weight gain occurs when energy intake (calories consumed) exceeds total daily energy expenditure for a prolonged period. An important function of physical activity is energy expenditure, which helps maintain energy balance and keep weight gain in check. Addressing the obesity problem requires examining both energy intake
(nutrition) and energy expenditure (physical activity). This study is focused on inadequate levels of physical activity—a major public health problem in its own right—and on the extent to which the built environment may play a role in fostering sedentary behavior.
ROLE OF THE BUILT ENVIRONMENT
The built environment is one of many variables thought to affect physical activity levels. The conceptual framework for this study (Figure ES-1) recognizes the complex relationships that affect the decision to be physically active. Much remains to be learned, however, about the relative importance of the individual (e.g., physical capacity, attitudes, preferences, time demands), the social context (e.g., social norms, support networks), and the physical environment as determinants of physically active behavior.
In contrast to the well-documented causal connection between physical activity and health, the role of the built environment in physical activity levels is a relatively new area of inquiry. The literature in this area is at an early stage of development, although it is growing rapidly. Results of this research to date, which has been largely cross-sectional, provide a growing body of evidence that shows an association between the built environment and physical activity levels. The science, however, is not sufficiently advanced to support causal connections or to identify with certainty those characteristics of the built environment most closely associated with physical activity behavior. Thus, the committee is unable to provide specific policy guidance, although it offers several recommendations for strengthening theory, research, and data that should provide a firmer basis for future policy making and intervention. The committee presents its consensus findings, conclusions, and recommendations in the following sections, which reflect the papers commissioned for this study, input provided at a workshop, numerous briefings provided to the committee at its meetings, and the expertise and judgment of its members.
Physical activity levels have declined sharply over the past half-century because of reduced physical demands of work, household management, and travel, together with increased sedentary uses of free time. Labor-saving technological innovations have brought comfort, convenience, and time for more leisure activities. They have also resulted in more sedentary lifestyles with adverse health effects for many Americans. Changes in land use and travel may also have contributed to the decline in physical activity levels, but the specific contribution that the built environment could make in rebuilding physical activity into the daily routine is not well understood.
The built environment can facilitate or constrain physical activity. The built environment can be structured in ways that give people more or fewer opportunities and choices to be physically active.
The characteristics of the built environment that facilitate or constrain physical activity may differ depending on the purpose of the activity. For example, ready access to parks and trails may facilitate walking for exercise; sidewalks and mixed-use development are likely to be more important to encourage walking for local shopping and other utilitarian purposes. The built environment can be changed in ways that increase opportunities for and reduce barriers to physical activity.
The relationship between the built environment and physical activity is complex and operates through many mediating factors, such as sociodemographic characteristics, personal and cultural variables, safety and security, and time allocation. Whether an individual is physically active is determined largely by his or her capacity, propensity, and willingness to make time for physical activity. For example, while public health surveys have found that on average physical activity levels decline with age, many senior citizens remain physically active. Individual behavior is also influenced by the social and physical environment (see Figure ES-1). For example, the social disorder and deteriorated physical condition of many poor innercity neighborhoods deter physical activity for many residents. These neighborhoods have some of the physical characteristics thought to be conducive to walking and nonmotorized transport—sidewalks, multiple destinations within close proximity, and mixed land uses—and indeed, low-income urban populations report high levels of walking for utilitarian trips. However, they also report low levels of discretionary physical activity. Crime-ridden streets, littered sidewalks, and poorly maintained environments discourage outdoor physical activity other than necessary trips. Time is another mediating factor and is cited by many as a reason for not being more physically active. For some (e.g., single parents, those holding two jobs), making time for physical activity is difficult. For others, particularly those who spend large amounts of leisure time on such sedentary pursuits as watching television, sedentary behavior may reflect the low priority given to physical activity. The role of time has not been well accounted for in examining the relationship between the built environment and physical activity.
The available empirical evidence shows an association between the built environment and physical activity. However, few studies capable of demonstrating a causal relationship have been conducted, and evidence supporting such a relationship is currently sparse. In addition, the characteristics of the built environment most closely associated with physical activity remain to be determined. Preliminary research does provide some evidence suggesting that such factors as access and safety and security are important for some forms of physical activity, such as walking and cycling, and for some population groups. However, the findings are not definitive because it is not known whether these characteristics affect a person’s overall level of physical activity or just his or her amount of outdoor walking and cycling. Furthermore, the literature has not established the degree of impact of the built environment and its various characteristics on physical activity levels; the variance by location (e.g., inner city, inner suburb, outer suburb) and population subgroup (e.g., children, the elderly, the disadvantaged); or the importance to total physical activity levels, the primary variable of interest from a public health perspective.
Weaknesses of the current literature include the lack of a sound theoretical framework, inadequate research designs, and incomplete data. The current state of knowledge in this area is limited in part by the lack of a sound theoretical framework to guide empirical work and inadequate research designs. As noted, most of the studies conducted to date have been cross-sectional. Longitudinal study designs using time-series data are also needed to investigate causal relationships between the built environment and physical activity. Studies that distinguish carefully between personal attitudes and choices and external influences on observed behavior are needed to determine how much an observed association between the built environment and physical activity—for example, in an activity-friendly neighborhood—reflects the physical characteristics of the neighborhood versus the lifestyle preferences of those who choose to live there. Appropriate measures of the built environment are still being developed, and efforts to link such measures to travel and health databases are at an early stage.
The built environment in place today has been shaped by longstanding policies and the practices of many decision makers (e.g., policy makers, elected officials, planners, developers, traffic engineers). Many existing development patterns have resulted from zoning and land use ordinances, design guidelines and funding criteria for transportation infrastructure focused primarily on motorized transportation, values and preferences of home owners and home buyers (e.g., suburban lifestyles, single-family housing), and racial and economic concentration of the poor and disinvestment in their neighborhoods. At the same time, the built environment is constantly changing as homes are renovated and new residences, developments, and office complexes are constructed.
Regular physical activity is important for health, and inadequate physical activity is a major, largely preventable public health problem.
The committee concurs with the strong and well-established scientific evidence linking physical activity to health outcomes and supporting reversal of the decline in overall physical activity levels as a public health priority. The connection between regular physical activity and health, although not the primary focus of this study, has clearly motivated interest in examining the built environment as a potential point of intervention to encourage more active behavior.
Built environments that facilitate more active lifestyles and reduce barriers to physical activity are desirable because of the positive relationship between physical activity and health.
Achieving this goal is challenging in a highly technological society with a built environment that is already in place and often expensive to change. Nevertheless, even small increases in physical activity levels can have important health and economic benefits. Moreover,
the built environment is constantly being renovated and rebuilt and new developments are being constructed; these changes provide opportunities to incorporate more activity-conducive environments. In the committee’s judgment, such changes would be desirable even in the absence of the goal of increasing physical activity because of their positive social effects on neighborhood safety, sense of community, and quality of life.
Continuing modifications to the built environment provide opportunities, over time, to institute policies and practices that support the provision of more activity-conducive environments.
The long-term decline in physical activity among the U.S. population has been the cumulative result of many changes; thus there are many opportunities for intervention. However, some interventions will be easier to effect than others. For example, formidable hurdles would have to be overcome to substantially modify long-standing policies, such as the current system of zoning regulations and land use controls that reflects the preferences of many suburban home owners and buyers, to allow greater density of development and more mixed land uses. Similarly, many barriers persist to ending concentrations of minority populations and underinvestment in poor neighborhoods and the accompanying social and economic isolation of the poor. More flexible and targeted approaches—context-sensitive design, special overlay districts, traffic calming measures, community policing—have a better chance of gaining support. Construction of new buildings and developments offers promising opportunities for creating more activity-friendly environments. A wider range of such environments should become available as more neotraditional communities2
prove financially successful and employers embrace more walking-friendly office complexes to encourage healthier workforces.
Opportunities to increase physical activity levels exist in many settings—at home, at work, at school, in travel, and in leisure. The built environment has the potential to influence physical activity in each of these settings.
Each setting is characterized by different environmental opportunities and constraints that could affect physical activity levels. In some neighborhoods, for example, residents walk for utilitarian purposes. Keeping these neighborhoods safe and providing desirable destinations should help reinforce and perhaps enhance this behavior. In other neighborhoods, walking for utilitarian purposes is limited. In these settings, recreational walking and cycling may offer the greatest potential for increasing physical activity in the daily routine. Of course, individuals can also obtain their daily physical activity by exercising at home. Most Americans spend the majority of their day at home, at work, and at school, and these are important but understudied locations for physical activity, particularly in view of guidelines suggesting that the daily 30-minute minimum of moderate physical activity can be accumulated in many locations and in small (10-minute) time increments.
Many opportunities and potential policies exist for changing the built environment in ways that are more conducive to physical activity, but the available evidence is not sufficient to identify which specific changes would have the most impact on physical activity levels and health outcomes.
Research has not yet identified causal relationships to a point that would enable the committee to provide guidance about cost-beneficial investments or state unequivocally that certain changes to the built environment would lead to more physical activity or be the most efficient ways of increasing such activity. Effective policies to this end are likely to differ for different population groups (e.g., children, youths, the elderly, the disadvantaged), for different purposes of physical activity (e.g., transportation, exercise), and in different contexts (e.g., inner city, inner suburb, outer suburb, rural).
Given the current state of knowledge and the importance of physical activity for health, the committee urges a continuing and well-supported research effort in this area, which Congress should include in its authorization of research funding for health, physical activity, transportation, planning, and other related areas.
Priorities for this research include the following:
Interdisciplinary approaches and international collaboration bringing together the expertise of the public health, physical activity, urban planning, and transportation research communities, among others, both in the United States and abroad.
More complete conceptual models that provide the basis for formulating testable hypotheses, suggesting the variables and relationships for analysis, and interpreting the results.
Better research designs, particularly longitudinal studies that can begin to address causality issues, as well as designs that control more adequately for self-selection bias.
More detailed examination and matching of specific characteristics of the built environment with different types of physical activity to assess the strength of the relationship and the proportion of affected population subgroups. All types of physical activity should be included because there may be substitution among different types. The goal from a public health perspective is an increase in total physical activity levels.
National public health and travel surveys should be expanded to provide more detailed information about the locations of physical activity and travel, which is fundamental to understanding the link between the built environment and physical activity in all contexts.
Geocoding the data on physical activity and health collected in large surveys, such as the Behavioral Risk Factor Surveillance System, the National Health and Nutrition Examination Survey, and the National Health Interview Survey, could help link these rich
data sets with information on the built environment and the specific locations where physical activity is occurring. Similarly, travel surveys, such as the National Household Travel Survey, as well as regional travel surveys, should be geocoded to provide more fine-grained geographic detail so researchers can link these surveys and diary data with characteristics of the built environment. In addition, data that reflect a more comprehensive picture of physical activity should be provided. For the public health databases, this means capturing more than leisure-time physical activity; for the travel databases, a more complete accounting should be provided of walking and other forms of nonmotorized travel. More reliable and valid measures of the built environment, both objective and subjective, are also needed. Technologies are available to help verify the accuracy of self-reported data automatically and objectively. Finally, a new database—the Bureau of Labor Statistics’ American Time Use Survey—provides an opportunity to track detailed types and durations of respondent activities in many locations. With the collection of extensive demographic and socioeconomic data on the respondents, the database offers researchers a more comprehensive picture of activities and time-use trade-offs by various subgroups of the population than has previously been available. Because the survey is new, opportunities exist to add questions related specifically to physical activity levels.
When changes are made to the built environment—whether retrofitting existing environments or constructing new developments or communities—researchers should view such natural experiments as “demonstration” projects and analyze their impacts on physical activity.
Numerous such opportunities exist, ranging from the construction of new, neotraditional developments to projects of the Active Living by Design program of the Robert Wood Johnson Foundation.3 To take advantage of these natural experiments, baseline data
must be collected. A “rapid-response” capability is needed so that timely funding can be made available to gather the appropriate data when opportunities arise.
Leadership of the Department of Health and Human Services and the Department of Transportation should work collaboratively through an interagency working group to shape an appropriate research agenda and develop a specific recommendation to Congress for a program of research with a defined mission and recommended budget.
An interagency approach is needed because the necessary research does not fall within the purview of any one agency. The committee recognizes that funding for research is currently being provided by the Robert Wood Johnson Foundation and encourages its continuation. Additional funding is needed to enhance research and data collection in several areas and provide a more solid foundation for policy making.
Federally supported research funding should be targeted to high-payoff but difficult-to-finance multiyear projects and enhanced data collection.
The highest priorities, in the committee’s judgment, include funding for multiyear longitudinal studies, a rapid-response capability to take advantage of natural experiments as they arise, and support for recommended additions to national databases. The federal government should supplement funding provided by foundations to ensure that this high-payoff research is conducted.
The committee encourages the study of a combined strategy of social marketing and changes to the built environment as interventions to increase physical activity.4
The research should be designed to study these approaches both separately and in combination so that the influence of individual
factors can be evaluated. To be effective, social marketing campaigns should be tailored to different population subgroups with relatively homogeneous characteristics and linked with other interventions involving the built environment for evaluation. For example, a social marketing campaign targeted to low-income, minority populations could be combined with a community policing effort to create safe havens for walking and studied for the effect on increasing physical activity levels in these communities. This more targeted approach should prove more effective than mass messages about the benefits of being physically active. Possible audiences include but are not limited to (a) subgroups of the population segmented by gender, age, income, and race; (b) public and private officials responsible for community design, development, safety, and public health; (c) transportation infrastructure planners and providers; and (d) private employers responsible for workplace design and employee information programs and incentives.
Universities should develop interdisciplinary education programs to train professionals in conducting the recommended research and prepare practitioners with appropriate skills at the intersection of physical activity, public health, transportation, and urban planning.
Ideally, new interdisciplinary programs should be developed with a core curriculum that brings together the public health, physical activity, transportation, and urban planning fields in a focused program on the built environment and physical activity. At a minimum, existing programs in public health, transportation, and urban planning should be expanded to provide courses related to physical activity, the built environment, and public health. Similarly, practitioners in the field—local public health workers, physical activity specialists, traffic engineers, and local urban planners—could benefit from supplemental training in these areas.
Those responsible for modifications or additions to the built environment should facilitate access to, enhance the attractiveness of, and ensure the safety and security of places where people can be physically active.
Even though causal connections between the built environment and physical activity levels have not been demonstrated in the literature to date, the available evidence suggests that the built environment can play a facilitating role by providing places and inducements for people to be physically active. Local zoning officials, as well as those responsible for the design and construction of residences, developments, and supporting transportation infrastructure, should be encouraged to provide more activity-friendly environments.
The committee believes that research on the relationship between the built environment and physical activity is at a pivotal stage. The number of investigators and studies is growing rapidly; interdisciplinary approaches are being encouraged; and technologies such as the Global Positioning System and geographic information systems, pedometers, and accelerometers are now available to provide and link more objective and detailed measures of both the built environment and physical activity. The committee also recognizes that policy prescriptions require a better understanding of causal connections than currently exists, as well as of the strength of these connections and their impact on population subgroups. In view of the importance of physical activity to health, the committee strongly urges that funding be provided to carry out its recommendations for conducting needed longitudinal studies, evaluating natural experiments, and enhancing data collection. To guide these efforts, the committee recommends a comprehensive approach focused on the individual and social as well as environmental determinants of physically active behavior.